1. Field of the Invention
This invention generally relates to conduits for use in subterranean wells where provision must be made for the severance of the conduit at a predetermined position and, more particularly, to concentric wall insulated tubing conduits and separation joints for these concentric wall conduits which are used to convey a heated fluid, such as steam, into the subterranean well.
2. Description of the Prior Art
It is often necessary to provide a means for severing a tubing string in a subterranean well at a predetermined location. Perhaps the most common situation in which separation of the tubing string at a precise location is necessary is the use of shear-out safety joints in tubing strings employing a safety valve. In such cases it may be necessary to sever the tubing string above the safety valve during an emergency. Shear-out safety joints are incorporated above safety valves so that in case of a disaster at the well head, the tubing string will separate above the safety valve permitting the safety valve to shut off the well.
The concentric tubing joint comprising the preferred embodiment of this invention can be used to provide a means of separating the tubing string at a desired location, especially when used in concentric walled insulating tubing string. In producing some subterranean wells, it is necessary to inject steam into an injection or producing well to increase the recovery of hydrocarbons by reducing the viscosity of the crude oil in the formation. One of the major problems in injecting steam into a subterranean production zone is that the heat transfer between the surface and the production zone is excessive when conventional well production tubing is utilized. Dual wall tubing structures having insulating material in the annulus between inner and outer walls welded at either end have been employed to reduce this heat loss. In U.S. Pat. No. 3,511,282 and in U.S. patent application Ser. No. 272,411, filed June 10, 1981 now U.S. Pat. No. 4,396,211, the inner wall of a concentric walled insulated tubing section is prestressed in tension to relieve the stresses in the tubular sections when heated steam is injected. The exterior walls of these concentric walled tubing sections are conversely prestressed in compression. In each of these tubular conduit members, means are provided for rigidly attaching the inner wall to the outer wall at opposite ends of the tubing. The conventional means of providing this rigid attachment is to weld the inner tubular ember to a bushing extending between the inner tubular member and the outer tubular member. In U.S. patent application Ser. No. 272,411, filed June 10, 1981, the inner tubing is flared so that a single weld may be employed to attach the inner conduit to the outer conduit at either end. Another dual wall conduit which employs a flexible bellows to permit differential expansion between the inner and outer tubular members is disclosed in U.S. Pat. No. 4,332,401. This dual wall device does not employ a tensile prestressed inner member with a compression prestressed outer member. Despite the structural differences between these separate concentric tubular conduit assemblies, it is quite apparent that each comprises a more complex and expensive structure than conventional oil well tubing. It is, therefore, more desirable that these tubular conduits and the welds incorporated therein not be damaged during operation.
The separation joint comprising the preferred embodiment of this invention therefore comprises a beneficial means for separating the tubing without risking damage to the more expensive insulated conduit. Perhaps the most common application of this separation joint would be its use in conjunction with the connection of insulated tubular conduit to a downhole packer. If for some reason a conventional packer employed in conjunction with an insulated tubular string cannot be released to permit retrieval of the tubular string, this separation joint would provide a means of releasing the tubing from the packer lodged in the well.
A separation joint intended for a similar purpose, but without the hermetically sealed annulus disclosed herein, is also disclosed in co-pending U.S. patent application Ser. No. 305,827 filed Sept. 28, 1981 now U.S. Pat. No. 4,423,778.